C 网络：Sendto() 返回 Errno 22，EINVAL

Question

我正在尝试使用第 2、3 和 4 层组件（即以太网、IP 和 UDP）从头开始生成数据包。我的套接字被配置为使用 SOCK_RAW 作为类型和 IPPROTO_RAW 作为协议，这样我的程序可以在以后用于发送不同的协议。我严格遵守 sendto(2) 手册页 (https://linux.die.net/man/2/sendto)，但是，我似乎仍然无法通过本地主机传输数据包。我的程序返回的错误是 22，即 EINVAL，表明我传递了一个不正确的参数。

我已经使用 Sockets sendto() returning EINVAL and Socket programming: sendto always fails with errno 22 (EINVAL) 来尝试解决我的问题。我什至将套接字切换为使用 SOCK_DGRAM 作为类型并使用 IPPROTO_UDP 作为协议以查看它是否与套接字有关（尽管我不相信这会提示错误论据回应）。

我的代码如下：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <netinet/ip.h>
#include <netinet/ether.h>
#include <netinet/udp.h>
#include <sys/socket.h>
#include <errno.h>

#define BUFFER 1024

int main(int argc, char *argv[]) {

        /* Socket Creation */
    int sockfd;

    if ((sockfd = socket(AF_PACKET, SOCK_RAW, IPPROTO_RAW)) == -1) {
            perror("Socket");
            exit(EXIT_FAILURE);
    }

    // This will hold all of the packet's contents
    char sendbuf[BUFFER];


    /* Address Stuff */

    // Specify Address
    in_addr_t address = inet_addr("127.0.0.1");

    // Specifying Address for Sendto()
    struct sockaddr_in sendto_addr;
    memset(&sendto_addr, 0, sizeof(sendto_addr));
    sendto_addr.sin_family = AF_INET;
    sendto_addr.sin_port = htons(9623);  // Make sure the port isn't contested
    sendto_addr.sin_addr.s_addr = address;

    // Size of whole packet
    size_t total_len = 0;


    /* LAYER 2 */
    // Ethernet Header Configuration
    struct ether_header *ether = (struct ether_header *)sendbuf;
    int i;
    for (i = 0; i < 6; ++i) {
        ether -> ether_dhost[i] = 0x00; // Temporary data fill
        ether -> ether_shost[i] = 0x00; // Temporary data fill
    }
    ether -> ether_type = ETHERTYPE_IP;

    total_len += sizeof(struct ether_header);


    /* LAYER 3 */
    // IP Header Configuration
    struct ip *ip = (struct ip *)(sendbuf + sizeof(struct ether_header));

    ip -> ip_hl = 5;
    ip -> ip_v = 4;
    ip -> ip_tos = 0;
    ip -> ip_p = 17;
    ip -> ip_ttl = 255;
    ip -> ip_src.s_addr = address;
    ip -> ip_dst.s_addr = address;  

    total_len += sizeof(struct ip); 


    /* LAYER 4 */
    // UDP Header Configuration
    struct udphdr *udp = (struct udphdr *)(sendbuf + sizeof(struct ether_header) + \
                                           sizeof(struct ip));

    udp -> source = 123; // Gibberish to fill in later
    udp -> dest = 321;   // Gibberish to fill in later
    udp -> check = 0;

    total_len += sizeof(struct udphdr);


    /* Giberrish Packet Data */
    sendbuf[total_len++] = 0x00;
    sendbuf[total_len++] = 0x00;
    sendbuf[total_len++] = 0x00;
    sendbuf[total_len++] = 0x00;


    /* Fill in Rest of Headers */
    udp -> len = htons(total_len - sizeof(struct ether_header) - sizeof(struct ip));
    ip -> ip_len = htons(total_len - sizeof(struct ether_header));


    /* Send Packet */  // ERROR OCCURS HERE
    printf("Sockfd is %d\n", sockfd);
    printf("Total length is %d\n", total_len);
    if (sendto(sockfd, sendbuf, total_len, 0, (const struct sockaddr*)&sendto_addr, \
        sizeof(sendto_addr)) < 0) {
            printf("Error sending packet: Error %d.\n", errno);
            perror("sendto Error");
            exit(EXIT_FAILURE);
    } 

    close(sockfd);

}

精确的控制台消息指出：

Sockfd is 3
Total length is 46
Error sending packet: Error 22.
sendto Error: Invalid argument

Answer 1

尤里卡！实际上是这一行：

if ((sockfd = socket(AF_PACKET, SOCK_RAW, IPPROTO_RAW)) == -1) {
        perror("Socket");
        exit(EXIT_FAILURE);
}

应该是

if ((sockfd = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) {
        perror("Socket");
        exit(EXIT_FAILURE);
}

为什么这解决了问题？好问题，很想看到有人对此主题的回应：P

C 网络：Sendto() 返回 Errno 22，EINVAL

C Networking: Sendto() returning Errno 22, EINVAL

c

sockets

networking

arguments

sendto